1. Field of the Invention
The present invention relates to a wiring substrate such as a ceramic multi-layer wiring substrate or other such substrate, a method of producing the same, and an electronic device including such a substrate.
2. Description of the Related Art
Recently, for electronic devices such as mobile communications equipment, computers, and other such devices, the required performance has been greatly increased, and the sizes of the devices have been greatly reduced. These improvements have been realized by the enhanced performance of semiconductor devices such as LSI or other such devices and by the high density wiring of wiring substrates on which the semiconductors are mounted. Ceramic multi-layer wiring substrates, which enable three-dimensional wiring and have a high reliability, are useful as these wiring substrates.
Moreover, with the improvements in performance and reduction in size of different types of semiconductor devices, it has become even more of a requirement that the ceramic multi-layer wiring substrates achieve connection via multi-terminals and with a small pitch, while having high wiring density and so that high frequency signals can be processed. In addition, in order to cope with the multi-terminal and small pitch connection, many fine land electrodes are disposed on the main surface of each ceramic multi-layer wiring substrate.
Referring to a ceramic multi-layer wiring substrate having a land electrode defining a soldering land, a technique for preventing solder from being excessively wetted and spread and by flowing out from a desired area, is known, in which a solder resist having a low wettability for the solder is formed to define a solder resist layer in an area where adhesion of the solder is undesired.
Conventionally, the solder resist layer for defining the soldering land is formed on the entire surface or almost the entire surface, excluding the solder land, of the ceramic multi-layer wiring substrate. For example, Japanese Unexamined Patent Application Publication No. 10-107442 discloses a ceramic multi-layer wiring substrate in which an overcoat layer is formed on the whole surface of the substrate, excluding the areas (soldering lands) that are intended to function as connection pads (corresponding to land electrodes) and terminal electrodes. Moreover, Japanese Unexamined Patent Application Publication No. 11-251723 discloses a ceramic multi-layer wiring substrate in which an insulation layer is formed on almost the entire surface of the substrate, excluding the area near the lower surface of the mounting part, in addition to connection pads for mounting a mounting part such as a capacitor, an oscillator, or other suitable component or element.
In general, the overcoat layer and the insulation layer for defining the solder formation areas on the ceramic multi-layer wiring substrate, are formed by adding and mixing ceramic powder or glass powder with an organic vehicle, and screen-printing the resulting insulation paste.
Here, a screen plate used for screen printing includes a mesh of knitted thin wires, and an emulsion for defining a paste-passing area. When the insulation layer is formed over a wide area excluding fine areas, the area ratio of the emulsion is small. Accordingly, in such a case, the emulsion tends to bend on the screen plate. Thus, in some cases, the insulation paste intrudes on the under side of the emulsion, which blurs the coated insulation paste.
For the purpose of mounting high-quality, small-sized semiconductor devices via multi-terminals and with small pitches, as described above, it is necessary to form many fine soldering lands on the main surface of a ceramic multi-layer wiring substrate. When the insulation layer and the overcoat layer is formed substantially on the whole surface excluding the soldering lands during the screen printing, the soldering lands tend to be spoiled because of blurring of the insulation paste. Thus, it is difficult to form fine soldering lands with high accuracy.